1. Field of the Invention
This invention lies in the field of physiological stimulating devices and, more particularly, demand-type cardiac pacing devices adapted for low power operation.
2. Description of the Prior Art
Cardiac pacers are, by this time, widely known and utilized by those skilled in the art. There is a widespread use of implanted pacers, as well as temporary pacers as used in cardiac care units. While the general success of cardiac pacers as such has been demonstrated by their adoption by cardiologists and physicians, there remains a pressing need for improvement of cardiac pacers, both in terms of the functions that the pacers are able to perform and the efficiency and reliability of such performance. As expected, when the sophistication of various functions performed by the pacer is increased, there is a concomitant increase in the required circuit complexity and correspondingly a decrease in reliability and efficiency. The fundamental need in the art is to provide a reliable pacer which incorporates the desired pacer functions in an efficient manner while optimizing usage of the power source and providing an integrity of operation which is substantially independent of the condition of the power source and other operating variables.
One of the primary functions which has gained acceptance in the field of cardiac papers is that of pacing in the demand mode, whereby the pacer is free running and delivers stimulus pulses when the patient's heart is not producing natural pulses, but which inhibits delivery of pulses when and as natural beats from the patient are detected. Other desirable functions include the adoption of hysteresis, whereby the effective timing of the pacer is varied when natural beats are detected, thereby allowing an increased range of natural operation, and observance of a refractory period following each delivered stimulus signal, whereby unwanted artifacts and spurious signals are not gated through for resetting of the pacer oscillator for a predetermined refractory period following each delivered stimulus signal. Yet another function which is considered essential in the art is that of detecting the presence of interference signal pickup, and adjusting the operation of the pacer during such interference to perform in a predetermined desired manner. All of these functions, as well as others, have been incorporated into various prior art devices, but the recurring need is for a pacer design which incorporates such functions and others in a manner that enables an extremely high degree of reliability and power-efficient performance.